This paper focuses on the realistic fire behavior of floor systems. An analytical approach based on the finite element method was used to model and analyze the composite floor system of a 10 story office building. A set of nonlinear analyzes were performed to evaluate the behavior of unprotected floor systems subjected to realistic design fires at three different locations, i.e., corner, exterior, and interior compartments. The results show that primary and secondary beams undergo large structural deformations and develop large axial forces at the beam ends during the heating and cooling phases of the fire. These axial forces are greater than the tension capacities of the beam end shear connections at ambient and elevated temperatures. This leads to the needfor structural fuse elements that can limit the axial force transfer to the beam shear connections. © 2009 ASCE.